Existing shipboard High Frequency (HF) transmit antennas, i.e. antennas transmitting waves between 1 and 30 MHz, cause major problems for proper mechanical integration on the ship. These problems are mainly due to the large extension of the antennas, which result in mechanical obstruction of on-board sensors and/or weapon systems. The height of these antennas also increases the risk of lightning strike. These problems are also related to high electromagnetic field strengths in the neighbourhood of the antennas, thus increasing the risk of radiation hazards to people and electromagnetic interferences (EMI) to other equipments. Moreover, the transmission efficiency is not optimal in a large part of the HF band due to a too low or too high antenna impedance. In addition, these problems are also related to high maintenance costs.
A conventional solution for providing a shipboard HF transmit antenna, consists in using a whip antenna, which is the most common example of a monopole antenna. Unfortunately, a whip antenna has many disadvantages. First, a shipboard HF transmit whip antenna is long, typically 10 meters. Furthermore, for a given frequency channel in the band, a whip antenna requires a tuning unit for proper impedance matching between the antenna itself, the generator and to the coax feed cable. Consequently, only one communication line can be used per whip antenna. When more communication lines are required, several 10 meters long whip antennas have to be arranged on the ship. This considerably increases the risk of EMI and radiation hazards. This also result in blocking of other equipment, which often causes serious performance degradation of shipboard radars and other sensors. In addition, the efficiency of such monopole antennas is low in a large part of the HF band.
Another conventional solution for providing a shipboard HF transmit antenna, consists in using towel bar antennas. Towel bar antennas are commonly used for so-called ‘Nearly Vertical Incident Skywave’ (NVIS) communication, which requires a high antenna gain at high elevation angles. Unfortunately, towel bar antennas have many disadvantages. First, towel bar antennas are not suitable for omnidirectional transmission at low elevation. Just as for the whip antenna, a tuning unit is required for impedance matching. Consequently, only one communication line can be used per towel bar antenna. When more communication lines are required, several towel bar antennas have to be arranged on the ship, thus increasing the risk of EMI and radiation hazards. In addition, the efficiency is low in a large part of HF band.
Yet another conventional solution for providing a shipboard HF transmit antenna, consists in using fan wire antennas. Fan wire antennas are commonly used for broadband transmissions. Even if the efficiency remains low in a large part of HF band, it is generally better in the lower part of the HF band than with whip or towel bar antennas. Unfortunately, fan wire antennas have many disadvantages. First, a fan wire antenna has to be quite large to optimise its efficiency in the lower part of the HF band. As a consequence, it generally has an extension above a large part of the ship, hereby dramatically blocking other equipments or leading to high risks of EMI.
In an attempt to overcome the aforementioned disadvantages, non-conventional concepts for HF antennas have been described, namely compact HF antennas and fractal antennas.
Compact HF antennas are antennas, of which length is less than a quarter the wavelength. For example, the spiral antenna, the magnetic loop antenna, the ExH antenna, the Crossed Field Antenna (CFA) or the Isotron antenna are compact HF antennas. Other examples are the helical whip antenna, the doublet antenna, as well as any small dipole or loaded dipole. Also for radio broadcast in the LF and MF bands, compact or so called ‘shortened’ antennas are used in some cases. Unfortunately, a compact HF antenna has also many disadvantages. In principle, the radiation efficiency of a compact HF antenna is extremely low, except for a very narrow frequency band. For this reason, compact HF antenna are often designed to be used in a fixed and quite narrow frequency band, even when it is labelled as a ‘broadband’ antenna. When a compact antenna is used for broadband transmission, it is accepted that the antenna efficiency is quite low.
Several types of compact antennas can be tuned, however the tuning of a compact HF antenna is critical, due to the extremely narrow bandwidth. The radiation efficiency remains still low, due to a bad matching of the real part of the impedance. Consequently, when more communication lines are required, several compact HF antennas have to be arranged on the ship, thus increasing the risks of EMI and radiation hazards.
Fractal antennas are a relatively compact type of antenna. Recently, it has been introduced a fractal antenna for naval HF communications. Unfortunately, a fractal antenna has also many disadvantages. Just as for the conventional and the compact HF antennas, the efficiency of fractal antennas is low in a large part of HF band due to a too low or too high real part of the impedance. Furthermore, just as for the monopole antenna, for a given frequency channel in the band, a tuning unit is required for proper impedance matching between the antenna itself, the generator and possibly to a coax feed cable. Consequently, only one communication line can be used per antenna. When more communication lines are required, several antennas have to be arranged on the ship, thus increasing the risk of EMI, radiation hazards and blocking of other equipment.
In an attempt to provide an HF antenna allowing easy mechanical integration on a naval ship, G. Marrocco and L. Mattioni recently described a naval structural HF antenna in their paper titled ‘Naval Structural Antenna Systems for Broadband HF Communications’ (IEEE transactions on antennas and propagation, vol 54, NO. 4, April 2006). The antenna described in this paper consists basically in a set of long vertical metal rods or wires, the set being so called “subradiator”, connected to the top of kind of an enlarged state-of-the-art mast or a large funnel. According to the authors, the principle of the structural antenna they describe is that of a folded monopole, where the subradiator is the radiating element and where the enlarged mast or the funnel acts only as a thick return wire. That is the reason why the subradiator must, in principle, be more than a quarter the wavelength to achieve reasonable efficiency. The performances of the described structural antenna are then optimised by forming an extra nested loop at the top of the subradiator and by arranging a set of impedance loads along the rods or wires. Unfortunatley, such an antenna still gives mediocre possibilities for integration. Indeed, a plurality of large subradiators are needed to achieve reasonable performances, since the described subradiators are typically 12 meters long. The large extension of the subradiators results in blocking or reflection of waves from and to other equipments, thus seriously degrading performances at a system level. The large extension of the subradiators also results in increasing the risk of EMI and radiation hazards. The use of subradiators peaking more than 12 meters high also increases the risks of lightning strike in the HF antenna. Moreover, even if the antenna offers the possibility for simultaneous transmissions, the number of frequency channels remains limited by the number of subradiators arranged around the enlarged mast or the funnel of the ship. Furthermore, each subradiator has to be connected to a separate power generator and tuning unit, which increase the amount of required equipment, the number of cables and thus also the complexity of the system integration.